1. Field of the Invention
This invention relates to a catalytic converter that is housed and fixed in a pipe that constitutes an exhaust gas discharge system.
2. Description of Related Art
A variety of efforts to reduce the burden on the environment are being made on a global scale in a number of industries. Of these, within the automotive industry, it goes without saying that gasoline-engine vehicles exhibiting excellent fuel economy are being developed, and the popularization of so-called eco-cars, as well as developments into further improving the performance of these eco-cars, are progressing on a daily basis. Examples of eco-cars include hybrid vehicles and electric vehicles. In addition to developments relating to such eco-cars, research into exhaust gas control catalysts, which control exhaust gases emitted by engines, is being actively carried out. Examples of these exhaust gas control catalysts include oxidation catalysts, three-way catalysts and NOx occlusion/reduction catalysts. The component that realizes the catalytic activity in these exhaust gas control catalysts is a noble metal catalyst such as platinum (Pt), palladium (Pd) or rhodium (Rh). Noble metal catalysts are generally used by being supported on a carrier consisting of a porous oxide such as alumina (Al2O3).
Exhaust gas discharge systems leading from a vehicle engine to a muffler generally include a catalytic converter used to purify exhaust gases. Engines may emit environmentally harmful substances, such as CO, NOx, unburned HCs and VOCs. In order to convert these harmful substances into acceptable substances, exhaust gases are passed through a catalytic converter obtained by disposing a catalyst layer, which is obtained by supporting a noble metal catalyst such as Rh, Pd or Pt on a carrier, on the surface of cells in a base material. CO present in the exhaust gas that passes through the catalytic converter is converted into CO2, NOx present in the exhaust gas are converted into N2 and O2, and VOCs present in the exhaust gas are burned to produce CO2 and H2O.
An example of a carrier used to support a noble metal catalyst is a CeO2—ZrO2 solid solution. CeO2—ZrO2 solid solutions are available as CZ materials or cerium oxide (ceria)-zirconia-based complex oxides. These are also available as co-catalysts. CeO2 is an essential component of three-way catalysts that simultaneously remove CO, NOx and HCs, which are harmful components present in exhaust gases, and is also an essential component of these co-catalysts. The oxidation number of Ce present in a co-catalyst changes to Ce3+ or Ce4+ depending on the oxygen partial pressure in the exhaust gas to which the CeO2 is exposed. Therefore, CeO2 present in a co-catalyst achieves the function of absorbing and releasing oxygen so as to compensate for an excess or deficiency of electrical charge and the function of storing oxygen (oxygen storage capacity (OSC)). Therefore, CeO2 can absorb and alleviate fluctuations in the atmosphere of an exhaust gas and can maintain an air-fuel ratio close to the stoichiometric air-fuel ratio in order to maintain the purification window of a three-way catalyst.
From perspectives such as reducing material risks of rare metals and the like and ensuring cost competitiveness, it is desirable to reduce the usage quantity of noble metal catalysts in such three-way catalysts. However, if the quantity of noble metal catalysts in a three-way catalyst is greatly reduced, catalyst activity is greatly reduced and OSC, low temperature activity and NOx purification performance in high temperature environments significantly deteriorate. That is, if the quantity of noble metal catalysts is greatly reduced, the number of active sites is greatly reduced, the number of catalytic reaction sites is greatly reduced, and purification performance therefore significantly deteriorates.
Among noble metal catalysts used in three-way catalysts, such as Pt, Pd and Rh, Rh exhibits the best NOx purification performance, but the market price per unit weight of Rh is the highest. In addition, it is understood that Rh exhibits a high OSC by being supported on a carrier that contains cerium oxide (ceria). Meanwhile, it is understood that there is a conflict whereby the NOx purification performance of Rh deteriorates as the quantity of cerium oxide in a carrier increases. Therefore, when using Rh as a noble metal catalyst in a three-way catalyst, it is desirable to produce a three-way catalyst in which both OSC and NOx purification performance are optimized.
The performance of a noble metal catalyst and carrier varies depending on the components used. Therefore, with regard to producing optimal three-way catalysts, diligent research has been carried out into zone-coated catalysts in which different components are provided on the upstream side and downstream side of a base material so as to effectively utilize the properties of each component.
Japanese Patent Application Publication No. 2012-040547 (JP 2012-040547 A) relates to such a zone-coated catalyst and discloses an exhaust gas purifying catalyst that has a base material, which forms a gas passage where an exhaust gas flows, and a catalyst layer formed on the base material. More specifically, the catalyst layer is constituted by a lower catalyst layer, a front upper catalyst layer and a rear upper catalyst layer. The lower catalyst layer is formed on the base material. The front upper catalyst layer covers the upstream side of the surface of the lower catalyst layer in the exhaust gas flow direction. The rear upper catalyst layer covers the surface of the lower catalyst layer further to the downstream side, in the direction of gas flow, than the front upper catalyst layer. The lower catalyst layer supports at least one of Pd and Pt, the rear upper catalyst layer supports Rh, and the front upper catalyst layer supports Pd. The Pd-supporting carrier of the front upper catalyst layer is a Y2O3-containing ZrO2 complex oxide. By using this configuration, it is possible to satisfactorily utilize the purification characteristics of the catalytic noble metals and also possible to increase the low temperature purification performance of the catalyst. In addition, by using a Y2O3-containing ZrO2 complex oxide, which has a low specific heat capacity and exhibits excellent heat resistance, as the carrier material for the front upper catalyst layer, it is possible to ensure heat resistance while improving the temperature increase properties of the catalyst and also possible to achieve sustainable catalyst warm-up properties.
Meanwhile, Japanese Patent Application Publication No. 2012-152702 (JP 2012-152702 A) discloses an exhaust gas cleaning catalyst that has a substrate, a lower catalyst layer, which is formed on the substrate and contains at least one of Pd and Pt, and an upper catalyst layer, which is formed on the lower catalyst layer and contains Rh. The exhaust gas upstream side of the exhaust gas cleaning catalyst has a region that does not include the upper catalyst layer. The lower catalyst layer consists of a front lower catalyst layer on the exhaust gas upstream side and a rear lower catalyst layer on the exhaust gas downstream side. The front lower catalyst layer includes an oxygen absorbing/releasing material. By using this configuration, it is possible to significantly inhibit particle growth by each of the catalyst metals supported in each of the catalyst layers, and particularly in the rear lower catalyst layer and upper catalyst layer that are on the exhaust gas downstream side. Furthermore, by providing a region in which the upper catalyst layer is not included in the exhaust gas upstream side, it is possible to increase the diffusibility of HCs into the interior of the front lower catalyst layer and also possible to promote HC purification in the front lower catalyst layer and therefore achieve satisfactory catalyst warm-up performance.
Furthermore, Japanese Patent Application Publication No. 2012-020276 (JP 2012-020276 A) discloses an exhaust gas purification catalyst in which a catalyst layer that constitutes the exhaust gas purification catalyst is constituted by a lower catalyst layer, a front upper catalyst layer and a rear upper catalyst layer. The lower catalyst layer is formed on a substrate. The front upper catalyst layer covers the upstream side of the surface of the lower catalyst layer in the exhaust gas flow direction. The rear upper catalyst layer covers the surface of the lower catalyst layer further to the downstream side, in the direction of gas flow, than the front upper catalyst layer. Here, the lower catalyst layer supports at least one of Pd and Pt. In addition, the front upper catalyst layer supports Pd and the rear upper catalyst layer supports Rh. The Pd bearing density in the front upper catalyst layer is 4.5 to 12 mass %. By using this configuration, it is possible to satisfactorily utilize the catalytic characteristics of the catalytic noble metals and also possible to increase the low temperature purification performance of the catalyst.
As shown above, many features relating to zone-coated catalysts exist. However, the inventors of this invention re-examined the configuration of zone-coated catalysts and succeeded in proposing a catalytic converter that exhibits excellent OSC performance and NOx purification performance.